The coupling impedance Zm is a parallel combination of R, L and C whose quality factor is low.

**CRO FORIMPULSE VOLTAGE AND
CURRENT MEASUREMENT**

The
coupling impedance *Zm* is a parallel
combination of *R*, *L* and *C* whose quality factor is low. The complex impedance *Zm* is given as

The
measuring impedance Zm is the impedance of a band pass filter which suppresses
harmonic currents depending upon the selected circuit quality factor Q, below
and above the resonance frequency f0 i.e., Zm will suppress all frequency
currents below and above its resonance frequency. The alternate is– 20 dB per
decade if Q = 1 and can be greatly increased.

Also, the
measuring circuit Zm performs integration of the PD pulse currents i (t) = I0 δ
(t).The above equation shows a damped oscillatory output voltage where
amplitude is proportional to the charge q. The charge due to the pulse i(t) is
actually stored by the capacitor C instantaneously but due to the presence of
inductance and resistance, Oscillations are produced. If these oscillations are
not damped, the resolution time of the filter will be large and proper
integration will not take place especiallyof the subsequent current pulses.
There is a possibility of over lapping and the results obtainedwill be
erroneous. The resolution time as is said earlier should be smaller than the
time constant τ of thecurrent pulse [*i*
(*t*) *I*0*e-t/*τ].The resolution
time or decay time depends upon the *Q*-factor
and resonancefrequency *f*0 of the
measuring impedance *Zm*. Let* Q *= 1 =* R*/* LC *. The voltage* v*0(*t*)
as shown in Fig. 6.28 can be obtained by writing nodal* *equation.

The
resolution time is about 10 μ sec and for higher values of *Q*, *T* will be still
larger. The resonance frequency is also affected by the coupling capacitance *Ck* and the capacitance *Ct* of the test specimen as these
contribute to the formation of *C*.
Therefore, the *R L C* circuit should
be chosen or selected according to the test specimen so that a desired
resonance frequency is obtained. The desired central frequency *f*_{0} or a band width around *f*0 is decided by the band pass amplifier
connected to this resonant circuit

These
amplifiers are designed for typically lower and upper cut off frequencies (– 3 *dB*) between 150 kHz and 100 kHz. This
band of frequency is chosen as it is much higher than thepower supply frequency
and also the frequency which are not used by broadcasting stations.

There
solution time becomes less than 10 μ sec. and hence proper integration of the
current pulse is made possible. However, the main job of the amplifier is to
increase the sensitivity of the whole measuring system. The time dependency of
the output voltage *v*0 (*t*) can be seen on the oscilloscope. In
the usual ellipse representation, the individual pulse *v*0 (*t*) are practically
only recognizable on vertical lines of different heights as one rotation of the
ellipse corresponds to one period of the supply system 20 m sec. for 50 Hz and
16.7 m sec. for 60 Hz supplies.

The
magnitude of the individual discharge is quantified by comparing the pulse
crest value with the one obtained from the calibration circuit as shown in Fig.
6.30. The calibration circuit consists of a voltage step generator *V*0 and a series capacitor *C*0. The charge *q* is simulated with no normal voltage applied to the PD testing
circuit. It is possible to suggest the location of the partial discharges in an
insulating material by looking at the display on the CRO screen.

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